Activate Grinding Station by tapping it and sacrificing Optimus Prime, Hero.
When Optimus Prime, Hero dies, its second ability triggers, returning it from your graveyard to the battlefield as Optimus Prime, Autobot Leader.
When Optimus Prime, Autobot Leader enters the battlefield, Grinding Station triggers, untapping Grinding Station.
Activate Grinding Station by tapping it and sacrificing Optimus Prime, Autobot Leader.
When Optimus Prime, Autobot Leader dies, Mikaeus triggers, returning Optimus Prime from your graveyard to the battlefield as Optimus Prime, Hero with a +1/+1 counter on it.
When Optimus Prime, Hero enters the battlefield, Grinding Station triggers, untapping Grinding Station.
Resolve both Grinding Station abilities from steps 1 and 4, causing a player to mill six cards.